Combustion of fossil fuels produces a number of undesirable pollutants including nitrous oxides (NOx). Environmental degradation attributable to NOx has become a matter of increasing concern, and therefore there is intense interest in suppressing NOx formation in fuel burning devices.
One of the principal strategies for inhibiting NOx formation is to burn a fuel-air mixture that is both stoichiometrically lean and thoroughly blended. Lean stoichiometry and thorough blending keep the combustion flame temperature uniformly low--a prerequisite for inhibiting NOx formation. One type of fuel injector that produces a lean, thoroughly blended fuel-air mixture is a tangential entry injector. Examples of tangential entry fuel injectors for gas turbine engines are provided in U.S. Pat. Nos. 5,307,634, 5,402,633, 5,461,865 and 5,479,773, all of which are assigned to the assignee of the present application. These fuel injectors have a mixing chamber radially outwardly bounded by a pair of cylindrical-arc, offset scrolls. Adjacent ends of the scrolls define air admission slots for admitting air tangentially into the mixing chamber. A linear array of equidistantly spaced fuel injection passages extends along the length of each slot. A fuel injector centerbody extends aftwardly from the forward end of the injector to define the radially inner boundary of the mixing chamber. The centerbody may include provisions for introducing additional fuel, or a fuel-air mixture, into the mixing chamber. During engine operation, combustion air enters the mixing chamber tangentially through the air admission slots while equal quantities of fuel are injected into the air stream through each of the equidistantly spaced fuel injection passages. The fuel and air swirl around the centerbody and become intimately intermixed in the mixing chamber. The fuel-air mixture flows longitudinally aftwardly and is discharged into an engine combustor where the mixture is ignited and burned. The intimate premixing of the fuel and air in the mixing chamber inhibits NOx formation by ensuring a uniformly low combustion flame temperature.
Despite the many merits of the tangential entry injectors referred to above, they are not without shortcomings that may render them unsatisfactory for some applications. One shortcoming is that the fuel mixture in the mixing chamber can encourage the combustion flame to migrate into the mixing chamber where the flame can quickly damage the scrolls and centerbody. A second shortcoming is related to the flame's tendency to be spatially unstable even if it remains outside the mixing chamber. The spatial instability is manifested by fluctuations in the position of the flame and accompanying, low frequency acoustic (i.e. pressure) oscillations. Although the acoustic oscillations may not be auditorially objectionable, their repetitive character can stress the combustion chamber and reduce its useful life. The injectors referred to above are ineffective at stabilizing the combustion flame and therefore may contribute to poor combustor durability.
The problem of flame ingestion into the mixing chamber can be mitigated by a uniquely contoured centerbody as described in copending, commonly owned patent applications Ser. No. 08/771,408 and Ser. No. 08/771,409, both filed on Dec. 20, 1996. The disclosed centerbody is aerodynamically contoured so that the fuel-air mixture flows longitudinally at a velocity high enough to resist flame ingestion and promote disgorgement of any flame that is ingested. Unfortunately, these desirable characteristics of the contoured centerbody can be impaired by the low velocity of fluid in the boundary layer adhering to the centerbody. This is particularly true if the slowly moving boundary layer fluid includes fuel as well as air. Moreover it has been determined that the contoured centerbody affects the fluid flow field within the mixing chamber in a way that disturbs the uniformity of the fuel-air mixture discharged into the combustor. As a result, the potentially damaging spatial instability of the combustion flame is exacerbated and the injector's full potential for inhibiting NOx formation may be compromised.
What is needed is a premixing fuel injector that inhibits NOx formation, spatially stabilizes the combustion flame outside the injector, effectively resists flame ingestion, and reliably disgorges any flame that migrates into the interior of the injector.